1. Field of the Invention
The present invention relates to a liquid crystal display device using a liquid crystal material having ferroelectricity or antiferroelectricity superior in a high-speed response.
2. Prior Art
The liquid crystal display device performs display by applying an electric field to the crystal material from the exterior so that the optical anisotropy of the material is changed to thereby convert an electric signal into a light signal.
FIG. 1 shows a structure of a general liquid crystal display device. The liquid crystal display device includes electrodes 103 and 104 formed on substrates 101 and 102 for driving a liquid crystal material. The electrodes 103 and 104 are disposed in such a manner that the respective electrodes 103 and 104 on the substrates 101 and 102 face each other. A liquid crystal material 105 is interposed between the substrates 101 and 102. The liquid crystal devices which utilize birefringence of the liquid crystal material have been widely known. In order to effectively utilize the birefringence of the liquid crystal material, one side of the respective substrates which is brought in contact with the liquid crystal material 105 is subjected to some orienting process 106 so that an optical axis of the liquid crystal material 105 is oriented in an intended direction.
Among the above-mentioned liquid crystal display devices, ones using the liquid crystal material having ferroelectricity or antiferroelectricity provide such an excellent characteristic that a response speed is approximately 1,000 times as high as that of the TN type or STN type liquid crystal display device.
As described above, in the liquid crystal device using the liquid crystal material having the ferroelectricity or antiferroelectricity, the surfaces of two substrates, which interpose the liquid crystal material therebetween and is to be in contact with the liquid crystal material are subjected to orienting process, and in the case of using the liquid crystal material, a uniaxial orientation is given to the liquid crystal material. As the orienting processes, there have been known a method of obliquely depositing SiO2 on two substrates interposing a liquid crystal material therebetween, a rubbing method of forming dielectric thin films on the surfaces of two substrates at the side of electrodes formed thereon as oriented films and of rubbing the surfaces of the dielectric films, and a method of applying an electric field or a magnetic field to a liquid crystal material from the exterior. Among these orienting processes, the rubbing method is superior in view of industry.
In the rubbing method, in general, an orientation film of 100 to 1000 xc3x85 is formed on at least one side of two substrates of the liquid crystal display device, which is in contact with the liquid crystal material, and the surface of the orientation film is subjected to the rubbing process, that is, the surface of the film is rubbed with cloth made of cotton, nylon or the like. The orientation films are made of organic component such as polyvinyl alcohol, nylon, polyimide or the like, or inorganic component such as silicon oxide.
When the liquid crystal has been optically uniaxially oriented by the rubbing process, the molecules of the liquid crystal are not arranged in parallel with the substrates interposing the liquid crystal.therebetween, but is obliquely oriented with a certain oblique angle. In general, this angle is called a pretilt angle. Conventionally, there has been considered that it is good to set the pretilt angle to 5xc2x0 or more in view of various points.
However, the liquid crystal using the liquid crystal material with ferroelectricity or antiferroelectricity has the following problems.
When the liquid crystal display device manufactured by orienting the liquid crystal material with ferroelectricity or antiferroelectricity is actuated by means of a square wave, there may occur a portion within a pixel where a light is leaked while displaying the dark state. Such a light leakage causes the contrast of the liquid crystal display device to be lowered.
In the liquid crystal display device of this type, in the case where the orientation state of the liquid crystal material is observed under the crossed Nicol through a polarization microscope, it is recognized that while the greater parts of the liquid crystal material are optically uniaxially oriented, however, defects in a line state or zigzag state occur partly.
Further, if the display mode is switched between the dark state and the bright state, it takes much time to obtain a given quantity of transmitted light after changing between the dark state and the bright state. Also, if any state is maintained as it is, there is a case where the quantity of transmitted light is changed. For example, there has been found such a phenomenon that the quantity of transmitted light of a pixel is gradually increased in the dark state.
Since the liquid crystal display device like this is unstable in the optical characteristics, it is not suitable for simple matrix drive for driving the display device by means of a pulse waveform and active matrix drive.
FIG. 2 shows a current-to-voltage characteristic of such a liquid crystal display device. Shown in the figure is a case where the liquid crystal material with ferroelectricity is used. The current-to-voltage characteristic has been measured by connecting a resistor of 100 kxcexa9 in serial to the liquid crystal display device, and by measuring a voltage drop of the resistor through an oscilloscope when applying a chopping wave of the 5 Hz frequency and the xc2x130 V voltage to the element. The current value in the figure represents a value resulting from dividing a voltage read by the oscilloscope by the above-mentioned resistance value. As shown in FIG. 2, the current component is classified into three parts, that is, a component 201 acting as a capacitor disposed between the electrodes of the liquid crystal display device, a current component 202 flowing when the spontaneous polarization of the liquid crystal material is inverted with the direction of the electric field being changed, and the other current component 203. The third current component will be hereinafter represented by a 2nd peak current.
As the second peak current is large, the initial transmissivity is not held and the optical characteristics become unstable. Therefore, the reduction of the 2nd peak current has been required.
The present invention has been made in view of the above-mentioned problems, and therefore it is an object of the invention to provide a liquid crystal display device with stable optical characteristics.
The above object of the invention has been achieved by provision of a liquid crystal display device, which comprises: first and second substrates on the surface of which electrodes are formed, respectively, a liquid crystal material with ferroelectricity or antiferroelectricity interposed between the first and second electrodes; and an orientation film disposed between the electrodes formed on the first and second substrates and the liquid crystal material and made of a polyimide based resin whose surface has been subjected to a process which gives optically uniaxial orientation to the liquid crystal material, which orientation film is capable of causing a nemaic liquid crystal to have a pretilt angle ranging between 1.6xc2x0 and 3.1xc2x0by contacting thereto.
Further, a liquid crystal display device according to the present invention comprises: first and second substrates on the surface of which electrodes are formed, respectively; a liquid crystal material with ferroelectricity or antiferroelectricity interposed between the first and second electrodes; and an orientation film disposed between the electrodes formed on the surfaces of the first and/or second substrates and the liquid crystal material and made of a polyimide based resin whose surface has been subjected to a process which gives optically uniaxial orientation to the liquid crystal material, in which a value of a polar term of surface tension on the surface of the oriented film ranges from 11 to 15 dyne/cm.
Further, in the above-mentioned structure, at least one of the first and second substrates comprises a substrate with a light transmission property.
Still further, in the above-mentioned structure, the first substrate is provided with a drive switching element thereon at each intersection of signal electrodes and scanning electrodes, and the second substrate is provided with an opposed electrode. The drive switching element comprises a thin film transistor for example.
The inventors have recognized some distinctive characteristic from the results of research.
The inventors manufactured a liquid crystal cell by changing the forming condition of an orientation film made of a polyimide based resin in the liquid crystal display device, measured a current-to-input voltage characteristic and a pretilt angle, and investigated the relationship between the pretilt angle and the second peak current.
The measurement of the pretilt angle has been conducted by the crystal rotation method which is an optical measuring method. Since this method is difficult to measure an optically biaxial material such as a ferroelectric liquid crystal material, an optically uniaxial liquid crystal such as a nematic liquid crystal was used and a measurement value of the pretilt angle of the nematic liquid crystal material in contact with the orientation film was used for measurement of the pretilt angle.
The results are shown in FIG. 3 (film thickness of 200 xc3x85 and baking temperature of 250xc2x0 C.). The liquid crystal cell having a small second peak current tends to hold an initial transmissivity. From the results shown in FIG. 3, as the pretilt angle of the liquid crystal cell is small, the second peak current is more decreased. That is, the initial transmissivity can be held when the pretilt angle is small.
The second peak current tended to be remarkably reduced particularly when the pretilt angle was set to 1.6 to 3.1xc2x0. At this time, the value of the polar term of the surface tension on the surface of the oriented film was 11 to 15 dyne/cm. The present invention utilizes this result to provide a liquid crystal display device with high contrast display and with stable optical characteristics.
The liquid crystal display device of the invention has been achieved by properly setting the forming condition of the oriented film and the optically uniaxial orienting process, that is, the rubbing condition.
As the forming condition of the orientation film made of a polyimide based resin, the sintering temperature of 200xc2x0 C. and the film thickness of 100 to 200 xc3x85 were proper.
In particular, when the whole film was not completely changed into imide and was not satisfactorily hardened, the film was considerably rubbed off by the rubbing operation. As a result, the orientation of the liquid crystal material has been dispersed, and therefore the sintering temperature is preferably set to 200xc2x0 C. or more.
As the rubbing condition, if rubbing is weak, the pretilt angle is increased whereby the orientation is deteriorated. In general, when the rubbing density is high, the strength of rubbing is increased. The relationship between the rubbing density and the pretilt angle is shown in FIG. 4 (the film thickness of 200 xc3x85 and the sintering temperature of 250xc2x0 C.). As can be seen from FIG. 4, there is a tendency that the pretilt angle is decreased as the rubbing density is increased. The rubbing density is a value obtained from the following equation 1.
L=N(1+2 xcfx80rn/60v)xe2x80x83xe2x80x83[equation 1]
where L is a rubbing density, N is the rubbing number of times, r is a radius of a rubbing roll (mm), n is a rotary speed of the rubbing roll (rpm), and v is a speed of a stage on which substrates are mounted (mm/sec).
For reducing the second peak, it is preferable to set the pretilt angle small, and therefore it is preferable to strengthen the rubbing treatment. However, if rubbing is too strengthened, there may occur a problem that the oriented film is peeled off, and therefore it is preferable to set the rubbing density to 100 to 500. If the pretilt angle is made smaller than 1.6xc2x0, the orientation film tends to be peeled off by the rubbing operation, whereas if it is made larger than 3.1xc2x0, the orientation defects are increased and the second peak is also increased.
In view of the above-mentioned film forming condition and the rubbing density, the pretilt angle is set to 1.6 to 3.1xc2x0. At this time, the value of the polar term of the surface tension on the oriented film is set to 11 to 15 dyne/cm.
In the liquid crystal display device in which a liquid crystal material with ferroelectricity or antiferroelectricity is optically uniaxially oriented, when the liquid crystal material is driven by applying an electric field to the material from the exterior, the spontaneous polarization is inverted by inversion of the electric field so that the direction of the longitudinal axis of the liquid crystal molecule is changed thereby performing the switching operation. After inversion of the electric field, the spontaneous polarization is aligned perpendicular to the substrates. As a result, because polarization occurs on the surface of the liquid crystal material, residual voltage is developed and an electric field occurs in the opposite direction inside of the liquid crystal material. Because of the electric field in the opposite direction, a torque is exerted in such a manner that spontaneous polarization is inverted, as the result of which the direction of the spontaneous polarization 502 is changed and deviate from the direction perpendicular to the substrates, as shown in FIG. 5.
Such a twisted orientation is of two-step response at the time of switching operation since the orientation of the spontaneous polarization which has been oriented at once is further changed. For this reason, when the current-to-input voltage characteristic is measured, two peaks occur.
With the structure of the invention, the above mentioned twisted orientation of the spontaneous polarization of the liquid crystal material is eliminated. Therefore, since the switching process is only of one step, the second peak current is decreased.
Therefore, by selecting appropriate film formation conditions and appropriate rubbing density, it is possible to obtain an optimum orientation control film where the pretilt angle and the surface tension of the orientation control film can be made within the foregoing ranges. Further, it is possible to remove an undesirable electric current component, i.e. the second peak current. Accordingly, the optical response can be improved and a stable display with a high contrast can be obtained.